1. OpenGL
    Interfaces and implementations of the OpenGL 3d graphics library
  2. opengl-extensions
    Extends cl-sdl to add bindings to opengl versions 1.3-2.0
  3. opengl-text
    opengl-text is a library for loading and displaying TrueType fonts as texture maps in OpenGL
  4. OpenMCL
    OpenMCL is called Clozure Common Lisp (CCL) now
  5. OpenMusic
    OpenMusic is a visual programming language based on Common Lisp and CLOS
  6. ops5
    OPS5 is a rule-based or production system computer language, notable as the first such language to be used in a successful expert system [...].
  7. opticl
    An image processing library for Common Lisp
  8. optima
    [deprecated by trivia, see link below] A pattern matching library
  9. Option-9
    A shoot-'em-up game written in Common Lisp
  10. optparser
    optparser is a Unix command-line options parser for simple shell scripts, geared towards ease-of-use and unambiguity